1. Field of the Invention
This invention relates to equipment for the Physical Vapour Deposition technique of magnetron sputter ion plating, that is to say for depositing materials in atomic and/or ionic form onto a receiving substrate by electrical discharge, for example to form coatings thereon.
2. Prior Art
Sputtering is a well-known process in which a cathode of a glow discharge is made a target for ions produced in the glow discharge in a surrounding low-pressure gas. The ions are accelerated towards the target by an electric field and the impact of them on the target displaces particles of the surface of the target; these particles are deposited on the surface of a suitably placed substrate to form the coating. It is known that the intensity of the glow discharge can be increased significantly by a magnetron effect, which causes the ionisation electrons to be trapped in a region where the electric field is crossed by an added magnetic field. This is the basis of magnetron sputtering, which gives deposition rates approximately ten times those from non-magnetron electrodes and also allows sputtering to take place at much lower gas pressures. Magnets are placed to produce lines of force passing across and above the surface of the target.
Ion plating is a well-known process in which a metal vapour produced in a vacuum system is deposited onto a substrate whilst, simultaneously, the substrate is bombarded with ions. The ion bombardment improves both the adhesion and the structure of the coating.
The metal vapour for ion plating can be produced by several techniques including sputtering. If sputtering is used as the vapour source in ion plating, the technique is called sputter ion plating. If magnetron sputtering is used as the vapour source in ion plating the technique is called magnetron sputter ion plating.
In ion plating, the ions which bombard the sample during deposition can be produced by several methods. In the basic ion plating method, the ions are produced in an abnormal glow discharge with the samples acting as the cathode. This is an inefficient process and typically less than 1 atom in 1000 is ionised in an abnormal glow discharge. The ion current to the samples is low, and is not sufficient to produce the dense coatings required in many applications, even though the samples are held at a high negative potential.
The ionisation can be increased in several ways. For example, the supply of ionising electrons can be increased by means of a hot filament and an electrode which is positive with respect to the filament, or a hollow cathode can also be used to provide a copious supply of electrons.
Rather than use additional filaments and electrodes to provide ionisation enhancement, it is convenient to use a vapour source that itself can act as a source of ionisation. A hot filament electron beam gun evaporator, a resistance heated crucible, and a simple diode sputter electrode are commonly used deposition sources that create little extra ionisation. On the other hand, hollow cathode electron beam guns, glow discharge beam guns and arc sources all produce intense ionisation at a level of over 50% ionisation without the need for additional ionisation enhancement devices, and consequently can be used to produce very dense coatings in ion plating systems.
Magnetron sputtering electrodes have been used in ion plating systems and they do increase the ionisation, but in the past this has not been sufficient to affect the coating structure and to produce dense coatings. A recent development has been the use of an unbalanced magnetron which has inner and outer magnets and in which the field strength of the outer magnets is much higher than the field strength of the inner magnets.
The ‘extra’ field lines leaving the outer magnets trap electrons escaping from the magnetron discharge and prevent them from drifting to the various earthed parts of the chamber. These electrons cause ionisation in the vicinity of the electrically biased substrate and the ions so formed are attracted to the substrate by the substrate bias, and the substrates receive a higher ion current than in a situation where the magnetrons are balanced. However, the intensity of ionisation may still be less than is desirable for the deposition of dense coatings, unless the outer magnets are made exceptionally strong.
It is thus clear that there are many ways of creating ions for sputtering or ion plating.
The aim of the invention is to provide an improved magnetron sputter ion plating system, with an increased intensity of useful ionisation.